Evolution has been described as 'a river that flows uphill' (Calvin 1987) or as a process that is 'climbing mount improbable' (Dawkins 1996). Such dynamics 'agains gravity' demand a force, or forces, to power the process. To explain the forces that shape the evolution of organisms, the metaphor of an artesian well will be used, where groundwater pressure pushes the water towards the surface. The well of evolution is powered by autocatalysis and variation, and its above-ground shape is the result of selection.

Driving forces

Have you ever wondered what is the driving force of evolution? Evolution requires a driving force, because it cannot 'just like that' lead to complex life forms. But force can this be? In fact, I will indicate that a combination of two forces is required. To explain this I will use the analogy of the artesian well. In an artesian well, high groundwater pressure pushes the water to the surface. And the water that wells up, is pulled back to the soil surface by gravity. It is the combination of these two forces, the pushing up and pulling down that shapes the waterflow coming out of an artesian well. A comparable picture can be scetched for the evolution of organisms. On the one hand, metabolic processes 'push' biomass production and offspring creation, while 'sloppiness' in the reproduction process automatically leads to variation. On the other hand, stressors can be viewed as 'pulling' at the result by having the most severe impact on individuals which are not well constructed to live under the prevailing conditions. The following feedback loop can now be observed: 1. there is a constant process of offspring production, 2. limitation of resources sets pressure on all organisms in the system, 3. which results in preferential survival of organisms with a high suvival capacity. 4. The result is that organisms with a high survival capacity (the best 'fitting' organisms) produce the most offspring. 5. The offspring with 'fit' genotype, will cause an increase in selection pressures. Which brings us bact to point 2. In principle, the average overall result is a net drive towards increasing compleity, which organisms need to maintain equal 'fitness'. This general trend is not contradicted by individual species of which the organisms are selected for simpler body plans, when this leads to higher fitness.

The force that powers reproduction

In an artesian well, groundwater pressure forces water to flow to the surface against gravity. But what pressure forces organisms into existence? As a (simple) example organism, we focus on a single celled organisms, a bacterium. A bacterium exists of a membrane surrounding a set of chemicals. In interaction, the membrane and chemicals are capable of (cellular) atuocatalysis. Autocatalysis occurs when a group of chemicals transforms energy rich substrate into new molecules that also belong to the group. Working together (without knowing it), the membrane and the autocatalytic set maintain the structure and functioning of the bacterium. The chemical activity of a bacterium can lead to three oucomes: 1. If the availability of energy-rich substrate is insufficient, the bacterium shrinks and starves, because it is not capable of creating enough new molecules to counteract natural breakdown processes. 2. If there is just enough enery-rich substrate available for maintenance, the bacterium survives withouth growth 3. If there is sufficient supply of substrate the bacterium produces abundant new material: it grows. At a given point growth may lead to the production of an offspring. Soon, mother and doughter will again produce an offspring. A chain reaction starts, and the numbers increase from 1 to 2, 4, 8, 16 etc. Option three thus implies an accellerating increase in the number of bacteria: the process 'explodes'. When 'food' is abundant, bacteria simply MUST grow. As long as food is abundant, evolution is powered by an explosive force: cell-based autocatalysis.

The forces that cause variation

The above shows that autocatalysis powers reproduction and the numerical increase of organisms. But numerical increase alone does not lead to evolution, which also depends on variation and selection. In a situation where every offspring has exactly the same physiology and construction as the mother, all individuals will remain the same and diversity remains constant. There are two forces that produce variation. The first force is the tendency of things to become disorderly or chotic. This is the force that makes a base-pair mutate randomly to another base-pair. Variation will increase in the absense of natural selection. This force has been called 'Biology's first law', or the 'zero-force evolutionary law' by McShea and Brandon (2010). The second force could be called 'sloppyness'. Sloppyness is the outcome of the balancing act of the organism between passive processes causing disorder, and active processes involved in the maintenance of order. This balance exists because it does not pay for organisms to achieve complete control over random change. Firstly, the energy needed for maintenance and repair of the organisms coding structure competes with the energy needed for growth and reproduction. Secondly, allowing a little bit of variation in the offspring creates opportunities for 'different' offspring to survive and reproduce when circumstances change and become unfavorable for the parents. The latter is a marked advantage in an unpredictable environment and as defence against diseases. Although sloppiness has advantages, too much of it has a prize too, because high variation will result in a large percentage of unadapted offspring, which is a disadvantage if the environment remains stable.

Stressors that lead to selection

There are various stresseors that can be viewed as forces causing selection. An important 'force' is the result of a 'race against the clock'. Organisms reproducing faster will generally outnumber those reproducing slowly. This force pushes organisms in the direction of speed and simplicity, because it generally takes more time to copy a complex/large organisation than a primitive/small one. The second force is caused by the race for the acquisition of resources. Either you have to be good at finding, exploiting and/or dominating resources, or you have to be good at defending yourself from being used as a resource. In many cases, these demands are best served by a large size and/or 'smart' construction. Also cooperation, which can be a very successful strategy for survival, demands complexity. Apparently, the gain in fitness that follows from complexity outweighs the disadvantage of slow reproduction, as most biomass on earth either takes the form of, or depends on, multicellular eukaryotic organisms.

A general definition of evolution

The balance between the push of variation and the pull of selection hints at a general minimal definition of evolution. The topic of how to create such a general definition of evolution and the implications for a general theory of evolution will be discussed in a later chapter.

Thermodynamics and the arrow of complexity

The arrow of complexity assumes that on average, the complexity of organisms increases over time. How can it be proven that nature follows this arrow, and that evolution leads to entities of ever increasing complexity, without that one uses teleological argumentation? According to the laws of thermodynamics, complexity can only form at the expense of entropy production. Nature had to obey this demand with every step on the complexity ladder, from quarks to animals with brains. The existence of animals, therefore, is a physical proof that the emergence of higher level operators represent a thermodynamically favorable process. Accordingly, (meta-)evolution, will occur in the direction of increasingly complex operators whenever conditions permit (complexity being measured as the number of first-next possible closures).